Add generate issue description to new issue page

What does this MR do and why?

Add description generator to New issue page, remove it from the Edit issue page for now. - https://gitlab.com/gitlab-org/gitlab/-/issues/406791

Works a bit differently to Generate MR description for a few reasons. We have less context, currently don't use the template, and has a separate modal for writing the prompt.

Context in https://gitlab.com/gitlab-org/gitlab/-/issues/406791/#note_1388521079

Screenshots or screen recordings

Screen_Recording_2023-05-18_at_20.47.13

How to set up and validate locally

1. Set up OpenAI API steps
2. Make sure feature flag is enabled:

   Feature.enable(:generate_description_ai)

3. In a public project, create a new issue
4. Click AI Actions dropdown and enter short summary in modal

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.